It is common in the wood construction industry to build roof and floor assemblies by applying continuous sheathing (1) such as plywood or corrugated steel panels to relatively small, closely-spaced repetitive secondary framing members (2). Secondary members are typically solid sawn 2″×4″ to 2″x12″ lumber spaced 12″ to 48″ O.C. The secondary members are then supported by larger, more widely spaced primary framing members (3). A wide variety of framing materials and frame spacings are used in primary framing members. Typically the primary members occur multiple times at more or less regular intervals. The secondary members are typically spliced (one member stops and another similar member takes its place and continues on) at each primary member. Alternately the secondary members may be spliced at every second or third primary member. FIG. 1 shows a typical roof plan with primary (3) and secondary (2) framing members. The primary and secondary members can occur repetitively along the length of the building.
The secondary members in the above system serve two functions. First, the secondary members support the sheathing (such as floors or roofs) and associated distributed loads. Second, the secondary members brace the primary members, increasing the load carrying capacity of the primary members by restraining out-of-plane buckling.
The secondary members in the above system are beams because the supported loads are applied perpendicular to the long axis of the members. Beams develop internal forces called shear and moment when loaded. Shear varies linearly over the length of a uniformly loaded beam. The moment is always zero at the free end of beams and varies parabolically over the length of uniformly loaded beams. A simple-span beam has a free end located at each support. FIG. 2 shows a uniformly loaded simple-span beam and the associated shear and moment distribution. FIG. 3 shows a double-span beam, a similar beam that is continuous over two spans. This continuity creates a negative moment region at the interior support ‘pulling’ the entire moment distribution downward as shown.
The largest moment (negative or positive) frequently controls the size of beam members required to support a structure. Techniques that reduce maximum absolute moment can allow the use of smaller, lower-cost members in place of larger, more expensive members that do not incorporate such techniques. Deflection is also a concern in sizing beams. Techniques that reduce deflection can also allow the use of smaller, less costly members in place of larger, more expensive members. The double-span beam reduces positive moment and drastically reduces deflection when compared to the similar simple-span beam. Also, the double-span beam has zero moment at four locations; once at each end and once at each side of the interior support.
A point of zero moment along the length of a beam is called a structural hinge. Structural hinges can occur as a result of a particular loading and support configuration such as in the two-span uniformly loaded beam shown. Or, an actual physical hinge can be placed in the beam to create a point with no moment capacity. A structural hinge connects the ends of two beams such that they must move together vertically but the ends are free to rotate independently. FIG. 4 shows the moment distribution with a structural hinge located as shown. The largest moment in FIG. 4 is only 0.66 of the simple-span moment. The maximum deflection is about half of the simple-span deflection.
Providing moment continuity across interior supports while including strategically located structural hinges may reduce both maximum absolute moment and maximum deflection thereby allowing the use of smaller, less expensive secondary members.
Primary framing members are frequently in compression as well as bending. Compression members can buckle out of plane at loads far below the member's actual strength. (FIG. 5). A compression member can buckle laterally forming half of a sine wave between points of lateral support. When this happens, the member also rotates about the support as it buckles. The distance between points of inflection on the sine wave is called the effective length. Shortening the effective length increases the force required to buckle a compression member. Adding a lateral brace shortens the effective length, increasing capacity. (FIG. 6). Preventing end rotation also shortens the effective length and increases the capacity of compression members. (FIG. 7).
A connection between a secondary framing member and a primary framing member that provides both lateral and rotational restraint of the primary framing member will shorten the effective length of the primary framing member, increasing its ability to support compressive forces and allowing the use of smaller, less expensive primary framing members.
Field installed structural hinges are not used to connect dimension lumber commonly used as secondary framing members in light-wood construction because no cost-effective means of making this connection is available. The methods currently used to connect secondary framing members to primary framing members do not provide rotational restraint of the primary framing members. There is a need for a cost-effective means of utilizing structural hinges and rotational restraints so that smaller framing members can be used, saving both resources and money.
Currently, it is common for secondary framing members to be spliced at the primary framing members. The secondary framing members are either set between or run over the primary framing members. Ledger boards or light-gauge steel hangers support the simple-span secondary framing members when they are set between the primary framing members. Usually simple lap splices are used when the secondary framing members run above the primary framing members. Sometimes butt splices are used. Often secondary framing members located above the primary framing members are simple spans. Sometimes the secondary framing members run continuously over two, three, or even four primary framing member spans. Long spikes or screws installed in holes drilled through the secondary framing members are frequently used to attach secondary framing members to primary framing members. A variety of light gage steel clips and wood blocking is also used. None of these solutions are optimal.
Solid sawn lumber up to 20′ long is readily available. Longer lengths are not. This means solid sawn lumber can only be used in the more efficient multi-span configuration when spans are 10′ or less. Simple-spans are inefficient because they do not include the moment and deflection reductions inherent with the development of a negative moment zone; lap splices require redundant material and result in offsets in the fastener lines; drilling holes for connection hardware can weaken the member at a high-stress location; and, because lumber is limited to 20′ maximum lengths, multi-span solutions require close spacing of the primary members, increasing both material and installation labor costs when compared to systems that enable fewer, larger primary members.
Engineers are aware of the advantages of continuous members and strategically located structural hinges. It is common practice when end splicing “I” shaped steel beams to use zero moment details such as the standard “End Plate Shear” or “Framed Beam” details. These details result in structural hinges because they support shear but do not carry moment (e.g., American Institute of Steel Construction).
Commercially-available, heavy, welded steel connection brackets are used to produce structural hinges in large wood beams (e.g., Simpson StrongTie HCA Hinge Connectors). Morton Buildings Inc. uses a structural hinge formed from two light-gauge steel plates that are pressed into the end of 2x4 secondary framing members at the factory. Assembly is completed with field-installed screws. None of these solutions provide a cost effective means of forming at the construction site the dozens of structural hinges required to connect secondary framing members in a typical wood frame structure.
Under the current state of the art, forming an end-to-end splice that results in a long continuous member requires precision shaping of the mating ends, specialized clamping equipment, exacting application of glue, and proper curing conditions (e.g., glulam beam fabricators). Continuous end-to-end splices must be made in a controlled environment. It is nearly impossible at a construction site. Further, transporting and handling long lumber is difficult.